Push critical resources for the initial URL route using <link preload>
and http/2.

Render initial route.

Pre-cache remaining routes.

Lazy-load and create remaining routes on demand.

Note http/2 push is a developing server technology and not available on
most hosts just yet.

Gatsby follows the PRPL architectural pattern. Gatsby sites render a static
HTML version of the initial route and then load the code bundle for the page.
Then immediately starts pre-caching resources for pages linked to from the
initial route. When a user clicks on a link, Gatsby creates the new page on
demand on the client.

This issue written at the start of Gatsby’s 1.0 work provides further background
on how Gatsby works to guarantee high performance.